Technologies are known in which an energy spectrum of radiation transmitted through a subject is measured to obtain transmissibility per unit energy specific to the material constituting the subject. An indirect conversion method and a direct conversion method are known as methods for measuring the energy of the radiation.
In the direct conversion method, the energy of the radiation is directly converted into an electrical charge, and measured as a signal output. In the indirect conversion method, the radiation is converted into visible light using a fluorescent material, such as a scintillator, and the light quantity of the visible light is measured to obtain the energy of photons. The indirect conversion method allows the size and the type of the scintillator to be varied, so that the energy of high-energy radiation can be received in a detector, and thus can be more flexibly used.
Interactions between X-rays and a substance include generation of fluorescent X-rays. Also in the scintillator used in the detector, the incident X-rays generate the fluorescent X-rays having intrinsic energy from atoms constituting the scintillator. If the generated fluorescent X-rays are emitted out of the scintillator, the apparent energy detected by the detector decreases by an amount corresponding to the amount of the emitted fluorescent X-rays, so that a correct measurement value is difficult to obtain, which is a problem.